GTBL042-16 GTBL042-Callister-v2 September 13, 2007 13:10
Revised Pages16.7 Forms of Corrosion • 683O 2O 2
O 2O 2O 2 O 2 O 2OH–OH–OH– OH– OH–Cl–Cl–Cl–
Cl– Cl–Cl– Cl–
Cl– Cl–Cl– Cl–Cl–
Cl–M+ M+
M+
M+M+M+M+
M M+M+
M+ +M+
M+
H+
H+
H+
H+
H+Na+Na+e–e–e–Figure 16.16
Schematic
illustration of the
mechanism of crevice
corrosion between
two riveted sheets.
(From M. G.
Fontana,Corrosion
Engineering,3rd
edition. Copyright
©c1986 by
McGraw-Hill Book
Company.
Reproduced with
permission.)The proposed mechanism for crevice corrosion is illustrated in Figure 16.16. After
oxygen has been depleted within the crevice, oxidation of the metal occurs at this
position according to Equation 16.1. Electrons from this electrochemical reaction are
conducted through the metal to adjacent external regions, where they are consumed
by reduction—most probably reaction 16.5. In many aqueous environments, the
solution within the crevice has been found to develop high concentrations of H+and
Cl−ions, which are especially corrosive. Many alloys that passivate are susceptible
to crevice corrosion because protective films are often destroyed by the H+and Cl−
ions.
Crevice corrosion may be prevented by using welded instead of riveted or bolted
joints, using nonabsorbing gaskets when possible, removing accumulated deposits
frequently, and designing containment vessels to avoid stagnant areas and ensure
complete drainage.Pitting
pitting Pittingis another form of very localized corrosion attack in which small pits or holes
form. They ordinarily penetrate from the top of a horizontal surface downward in a
nearly vertical direction. It is an extremely insidious type of corrosion, often going
undetected and with very little material loss until failure occurs. An example of pitting
corrosion is shown in Figure 16.17.
The mechanism for pitting is probably the same as for crevice corrosion in that
oxidation occurs within the pit itself, with complementary reduction at the surface.
It is supposed that gravity causes the pits to grow downward, the solution at the pit
tip becoming more concentrated and dense as pit growth progresses. A pit may be
initiated by a localized surface defect such as a scratch or a slight variation in compo-
sition. In fact, it has been observed that specimens having polished surfaces display
a greater resistance to pitting corrosion. Stainless steels are somewhat susceptible
to this form of corrosion; however, alloying with about 2% molybdenum enhances
their resistance significantly.